How to model depressional area in HEC-HMS

[meehow09]

For a project I'm working on, a subbasin 1A flows to a depression which then overflows to an existing culvert. I am unsure what is the correct way to model this. I tried both these ways:

Method 1. I modeled the depressional area as a reservoir with a spillway outlet. I had to add an arbitrary orifice outlet at the bottom of the depression with diameter 0.001 to effectively produce 0 CSS discharge under the overflow elevation.

Method 2. I divided the depressional storage by the drainage area and included it as an initial abstraction over the entire subbasin 1A under the Simple Surface input. I made a separate reservoir with the spillway set at the depression overflow elevation to model discharge flows.

However, I get different discharges for both methods. Method 1 results in an increased discharge over method 2.

 

[pinkpig]

Method 2 seems kind of double dipping the impact and that's why you got a lower discharge. I would go with method 1,and also make sure since you are modeling the depression as reservoir, when you calculate your basin hydrological parameters, don't include any factors from the reservoir.

 

[mdrehansk]

hi,
if you know the size of the depression or volume then use storage discharge curve in reservoir..do not apply any arbitrary outlet or orifice..follow below steps
1- calculate what is the peak discharge from 1A including depression storage area...but donot use reservoir in this run
2- once know the peak discharge...now apply reservoir with storage discharge curve.
3- in storage discharge curve if apply maximum discharge at full volume for example: say your full volume of depression is 10000m3 and peak discharge is 3m3/s.

Volume Discharge
0 0
500 0
5000 0
8000 0
10000 3
11000 3

by this the pond will act as natural pond and you will get exact outflow discharge from the pond.